A 7-dimensional Riemannian manifold (M,g) is called a G_2 manifold if the holonomy group of its Levi-Civita connection of g lies inside G_2. In this talk, I will first give brief introductions to G_2 manifolds, and then discuss relations between G_2 and contact structures. This is a joint work with Hyunjoo Cho and Firat Arikan.